Apparatus for the method of liberating and removing fibrous material from a mineral ore

ABSTRACT

An apparatus for and a method of removing fibrous material from a mineral ore. In the described embodiment, the ore is first crushed in a suitable crushing apparatus to the proper feed size for a first grinding mill, normally a rod mill. The first grinding mill is so adjusted or designed as to grind the ore fed thereto to the coarsest grind at which most of the fibrous material is liberated from the rest of the ore. The fibrous material is a relatively coarse size when liberated at this grinding level. The ground material discharged from the first grinding mill is then fed as a slurry to a screening device having openings which are so sized as to retain on the screen as &#39;&#39;&#39;&#39;oversize&#39;&#39;&#39;&#39; the liberated fibrous material, but which pass the defibered slurry through the screen as &#39;&#39;&#39;&#39;undersize.&#39;&#39;&#39;&#39; The defibered slurry is then passed to an additional grinding stage or stages in which the valuable or desired mineral is liberated to a desired degree from the defibered gangue material.

limited States Patent [191 Kjos et a1.

APPARATUS FOR AND METHDD OF LIBERATING AND REMOVING FIBROUS MATERIAL FRGM A MINERAL ORE Inventors: David M. Kjos, Wauwatosa; Chester A. Rowland, Jr., Shorewood, both of Wis.

Allis-Chalmers Corporation, Milwaukee, Wis.

Nov. 23, 1971 Assignee:

Filed:

Appl. No.:

References Cited UNITED STATES PATENTS 9/1928 Dolbeard ..241/4 3/1950 Crockett ..241/4x [451 Apr. 17, 1973 3,186,647 6/1965 Bacher ..24l/4 Primary ExaminerGranvi1le Y. Custer, Jr. Attorney-Robert C. Sullivan et a1.

[ 57] ABSTRACT An apparatus for and a method of removing fibrous material from a mineral ore. In the described embodiment, the ore is first crushed in a suitable crushing apparatus to the proper feed size for a first grinding mill, normally a rod mill. The first grinding mill is so adjusted or designed as to grind the ore fed thereto to the coarsest grind at which most of the fibrous materia1 is liberated from the rest of the ore. The fibrous material is a relatively coarse size when liberated at this grinding level. The ground material discharged from the first grinding mill is then fed as a slurry to a screening device having openings which are so sized as to retain on the screen as oversize the liberated fibrous material, but which pass the defibered slurry through the screen as undersize. The defibered slurry is then passed to an additional grinding stage or stages in which the valuable or desired mineral is liberated to a desired degree from the defibered gangue material.

31 Claims, 2 Drawing Figures 12 WATER INHfiJJ %F1BER LADEN SLUFZRY I5 REJECTED F IBERS DEFIBERED SLURRY EXCESS WATER OPEN cuacunvmw All FRNATIVF CLOSED c/Rcun PATH 8 OVERSIZE LCLOSED CIRCUIT PRODUCT (ALTERNATIVE) '1?) FURTHER PROCESSING PATENTEBAPR I 71975 3. 7, 349

SHEET 1 UF 2 WATER IN I i FIBER LADEN SLURRY OPEN CIT2CUIT PATH l 23 CLAssIEIER 24.

ALTERNATIVE CLOSED/ I J CIRCUIT PATH ]8 OVERSIZE LCLOSED CIRCUIT PRODUCT 22 2 (ALTERNATIVE) TO FURTHER PROCESSING PATENTEDAFR] 7197s 7 7, 49

SHEET 2 [1F 2 WATER )N Q FIBER LADEN SLURRY REJECTED FlBERS f 9" 2 DEFIBERED AB SLURRY 1 OVERSIZE CLASSIEIER UNDERSIZE TO FURTHER PROCESSING 1 APPARATUS FOR AND METHOD OF LIBERATING AND REMOVING FIBROUS MATERIAL FROM A MINERAL ORE BACKGROUND OF THE INVENTION Certain metallic mineral ores such as, for example,

certain deposits of nickel sulphide associated with serpentine gangue, contain a substantial amount of naturally occurring fibrous minerals or material such as asbestos. These fibers vary in amount and occurrence quantity and remains in the ore stream during the normal processes of comminution, classification, and concentration, the presence of the fibrous mineral causes operating problems such as high cost, and circuit inefficiencies (i.e., high grinding unit power and poor classification). These mineral fibers tend to remain long and stringy in nature, relative to the size of the rest of the ore particles at a given degree of grind of the ore. The fibers build up on the discharge grates of ball or pebble mills, on trommel screens, pumps, classifiers such as spirals, rakes, and cyclones, and in concentration machines, such as floatation cells and magnetic separators or the like. These mineral fibers will tend to build up even in an open chute where there might be a minor obstruction such as a bolt head or a weld big enough to catch one or two fibers. Once several fibers are caught or accumulate they begin to cause an obstruction to the flow of the material.

To the best of our knowledge, in processing mineral ore containing fibrous material, the sole criterion applied in the prior art has been to arrange the grinding circuit and the components thereof so as to best effectuate the separation of the desired or valuable mineral from the gangue material, without consideration to the grinding requirements for separating the undesirable fibrous material from the ore. As a result of this approach to the grindingprocess, in a typical case, the ore is overground" in the initial grinding as far as the requirements for liberating the fibrous material is concerned. That is, where the sole consideration is the separation of the valuable mineral from the gangue 'materiaLthe ore is typically more finely ground than required for liberation of the fibers. This finer grind will liberate the fibers, but, in so doing, will cause breakage of the fibers into smaller pieces. If ore containing such liberated but broken fibers is subjected to the action of a classifying device of the cyclone type, for example, which depends for its sizing operation of the mass of the particles being sized, it is inherent that the product stream exiting from the cyclone will include many undesired fibers which respectively have the same weight as the particles of the mineral which it is desired to separate out in the product stream, thereby contaminating the product stream with the fibers and causing the adverse effects previously described.

STATEMENT OF THE INVENTION Accordingly, it is an object of the present invention to provide an apparatus .for and a method of eliminating naturally occurring fibrous materials or minerals from a mineral ore stream at an early stage in the processing of the ore stream.

It is another object of the invention to provide a method of eliminating naturally occurring fibrous materials or minerals from an ore stream at a controlled point in the processing of the ore stream.

It is a further object of the invention to provide an apparatus for and a method of processing mineral ore of the type containing naturally occurring fibrous materials or minerals, so as to increase the efficiency of the ore processing operation.

It is a still further object of the invention to provide an apparatus for and a method of processing mineral ore of the type containing naturally occurring fibrous materials or minerals, whereby to reduce operating costs, increase availability of the total processing circuit, and increase the efficiency of comminution and concentration.

In achievement of these objectives, there is provided in accordance with an embodiment of this invention an apparatus for and a method of processing mineral ore of the type having undesirable fibrous material, such as asbestos, associated with the ore.

In accordance with an embodiment of the apparatus and method, the ore is first crushed in a suitable crushing apparatus to the proper feed size for a first grinding mill, normally a rod mill. The first grinding mill is so adjusted or designed as to grind the ore fed thereto to the coarsest grind at which most of the fibrous material is liberated from the rest of the ore. The fibrous material is of relatively coarse size when liberated at this grinding level. The ground material discharged from the first grinding mill is then fed as a slurry to a screening device having openings which are so sized as to retain on the screen as oversize" the liberated fibrous material, but which pass the defibered slurry through the screen as .undersize. The defibered slurry is then passed to an additional grinding stage or stages in which the valuable mineral is liberated to a desired degree from the defibered gangue mineral.

Further objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT The metallic mineral ore to be processed such as nickel sulphide associated with a serpentine gangue mineral which includes a fibrous mineral content such as asbestos, is prepared for subsequent grinding by first crushing the ore in a conventional crusher device or the like generally indicated at to a feed size suitable as the feed input for the grinding mill 12. The input feed size to grinding mill 12 as produced by crusher 10 may, for example, be minus three-fourths inch size. Since in practicing the present invention the fibrous material is normally liberated from the rest of the ore at a relatively coarse grind of the ore, grinding mill 12 is normally a rod mill which, as is well known in the art, is intended for grinding mineral ore to a coarser grind than that provided by a ball mill. Water is preferably added to the inside of mill 12 to provide a wet grinding process, causing the discharge from grinding mill 12 to be in the form ofa slurry.

The degree of grind produced by the grinding mill 12 is so selected as to be substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore. By liberating the fibrous material from the rest of the ore at the coarsest possible grind at which most of the fibers are liberated, fiber breakage is minimized and the liberated fibers thus obtained are inherently of as coarse a size as is possible in the liberated state of the fibers at any grinding level. This grinding level or point for optimum liberation of the fibers can be established by bench scale (laboratory scale) test work. For example, the coarse ore in the grinding mill 12 may be milled to a grind at which approximately 80 percent of the material emerging from the discharge end of grinding mill 12 passes through a 14 mesh screen. This value is given only by way of example as a possible typical value of the coarsest grind at which most of the fibers will be liberated from the ore.

Due to the normal orientation in which the fibers present themselves to the apertures in a screening device (to be described), the length of the fibers is a major factor, although not the only factor, in determining the coarseness of the fibers relative to the screen member. The fact that the fibers when initially liberated from the ore, particularly when liberated at the coarsest possible grind at which most of the fibers are liberated, are normally in groups or bundles, and therefore present themselves to the apertures of the screening device as such groups or bundles, is another factor contributing to coarseness of the fibrous material at this grinding level.

As is well known in the mineral processing industry, the degree of grind of the output of a given fixed size rod mill having a nominal grinding rating of 10 mesh, for example, may be adjusted within limits by variation of one or more parameters such as the following:

. Variation of feed size of ore being processed;

2. Variation of the number of rods in mill;

3. Variation of weight of grinding media (in the case ofa rod mill, the weight of the rods);

The rotary speed of the mill can be varied within limits to thereby vary the degree of grind of the output product;

5. Variation of the amount of water added; and

6. Variation in the rate of input feed per unit time to the mill of the ore being processed. If the grinding mill 12 is especially designed for its function of liberating fibrous material from the rest of the ore, such a mill would be designed to give the desired grind at rated horsepower, rated speed, etc., without the necessity of major adjustments in any of the previously listed variables.

The slurry discharged from mill 12 passes through a screening device 14 which may be a stationary screen such as a stationary sieve bend, or alternatively, a vibrating screen, for example. The openings of the screening device 14 are sized to permit the defibered slurry to pass through the screen as undersize and to retain the liberated coarse stringy fibers on the screen as oversize. The oversized material can then be rejected or processed further if it occurs in sufficient quantity.

A suitable dewatering device 17 such as a dewatering classifier, is preferably but not necessarily, interposed in the flow path between the discharge end of screen 14 and the input end of the second stage grinding mill 16 to remove excess water from the defibered slurry which has passed through screen 14 and before it passes into grinding mill 16. The excess water exits from the dewatering device 17 via the path indicated at 19 in FIG. 1.

The removal of the excess water by the dewatering device 17 is particularly desirable where the second stage grinding mill 16 is used for open-circuit grinding as indicated in full line in FIG. 1 rather than being used as a second stage recycling grinding mill as indicated in phantom line in FIG. 1, although the dewatering device may also be used when mill 16 is in the closed grinding circuit shown in phantom line in FIG. 1.

Dewatering classifiers are per se well known in the art. For example, the apparatus shown on page 8-46 (FIG. 32) and described on page 8-47 of Handbook of Mineral Dressing by Arthur F. Taggart, New York, John Wiley & Sons, lnc., London: Chapman & Hall, Limited, Copyright 1945, Fourth Printing, September 1950, may be used as the dewatering device 17. Also, a device of the type shown in FIG. 8, page 10-07 and described on page l0-07, of the aforesaid Handbook of Mineral Dressing" may be used as a dewatering classifier.

The dewatering device 17 may be adjusted to provide a controlled degree of dewatering of the defibered slurry entering second stage grinding mill 16. A grinding mill such as mill 16, operating on wet process grinding, runs better and the mill circuit can be better con: trolled if the input to the mill is in the form of a fairly thick slurry. Water from a separate source-can be then added in controlled amounts to the dewatered slurry on the interior of mill 16 if required to provide proper solids-liquid ratio in mill 16.

A further reason for possibly needing a dewatering device 17 in the grinding circuit as shown in FIG. 1 is that excess water may possibly be used as a transport medium to transport the material discharged from the first grinding mill 12 via the path 15 to the screening device 14 in which case the dewatering device l7 may be needed to remove the excess water used as the transport medium.

The defibered slurry which has passed through the screen 14 and is now relatively free from the liberated fibers and which has preferably but not necessarily had excess water removed therefrom by a device such as the dewatering device 17, then passes to a second grinding stage which in the illustrated embodiment comprises a conventional grinding mill such as that indicated at 16 which may be a ball mill, for example. The grinding capability of the grinding mill 16 is so selected or adjusted as to provide an output grind which is suited to the requirements of the particular defibered mineral ore, whereby the valuable mineral (or the particular mineral desired) such as nickel sulphide, for example, is liberated most efficiently to a desired degree from the defibered ore. Thus, for example, the circuit output of ball mill 16 may have a grind characteristic such that 85 percent by weight of the output passes through a 150 mesh screen.

The ground material discharged from the second grinding mill 16 may pass in an open circuit path along discharge path 22, indicated in solid line in FIG. 1 of the drawing, to further processing.

Alternatively, as shown in phantom line in FIG. 1 of the drawings, the ground product being discharged from the second stage grinding mill (or ball mill) l6 instead of following the open circuit path shown in full line in FIG. 1 may instead pass as shown in phantom line 23 to a classifier 18 which separates the oversized product from the undersized product. The undersized product from classifier 18 passes via the path indicated at 20 back into the discharge flow path 22, while the oversized product from the classifier 18 is recycled through the flow path indicated at 24 back into the input or feed end of the second grinding mill 16 where the oversized product is subjected to further grinding.

It will be understood that the open circuit second stage grinding circuit shown in full line in FIG. 1 and the closed circuit second stage grinding circuit shown in phantom line in FIG. 1 are alternative circuit arrangements which may be used.

Referring now to FIG. 2, there is shown a modified ore flow diagram which differs from the closed grinding circuit shown in phantom line in FIG. 1 in the use of a modified closed circuit arrangement for the processing of the defibered slurry. As shown in FIG. 2, the mineral ore having the fibrous content is prepared for subsequent grinding by first crushing the ore in the conventional crusher device indicated at 10' to a feed size suitable as the feed input for the grinding mill 12'. The

' ore is ground in grinding mill 12 to liberate the fibrous material from the ore and passes to screening device 14', all as previously described in connection with the embodiment of FIG. 1. The fibers are separated from the slurry by screening device 14' as previously described and the defibered slurry passes into a classifier device 18' which separates the defibered slurry into an oversized material and into an undersized material. The undersized material exits from the classifier device 18' via line 22' and passes to further processing as required by the natureof the ore. The oversized material exits from classifier device 18 via line 24' and passes into the inlet end of a conventional grinding mill 16' which may be a ball mill, for example. The output product of mill l6 exits via line 23' and is recycled back into the inlet line to classifier device 18', with the recycled: material from the grinding mill 16 again being subjected to the classifying action of the classifier device 18'.

It will be seen that the second stage grinding circuit shown in the diagram of FIG. 2 is generally similar to that shown in the closed circuit arrangement shown in the phantom line in FIG. 1 of the drawing except that in the arrangement of FIG. 2, the classifier 18' is positioned upstream of the second stage grinding mill 16' whereas in the closed circuit of FIG. 1, the classifier 18 is located downstream of the second stage grinding mill 16.

In the specification, all references to mesh sizes are to Tyler mesh sizes which is a well known standard in the mineral processing industry. While only one grinding stage has been shown and described for grinding the defibered ore downstream of the screening device 14 or 14 in the embodiments of FIGS. 1 and 2, it will be understood, of course, that a pluralityof grinding stages may be used downstream of the screening device for grinding the defibered ore if required.

From the foregoing detailed description of the present invention, it has been shown how the objects of the invention have been obtained in a preferred manner. However, modifications and equivalents of the disclosed concepts such as readily occur to those skilled in the art are intended to be included within the scope of this invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for processing a mineral ore of the type having a desired mineral associated with a gangue material and afibrous material associated with and constituting part of the ore, comprising a first grinding means adapted to receive said mineral ore and to grind said mineral ore to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore, a screening device adapted to receive the ground ore including the liberated fibrous material discharged from said first grinding means, said screening device having openings so sized as to retain on said screening device as oversize the liberated fibrous material, and to pass through said screening device the defibered ore, and additional grinding means adapted to receive the defibered ore passing through said screening device, said additional grinding means being effective to liberate said desired mineral to a desired degree from the defibered ore.

2. An apparatus as defined in claim 1 in which said mineral ore is nickel sulphide associated with serpentine gangue.

3. An apparatus as defined in claim 1 in which said fibrous material is asbestos.

4. An apparatus as defined inclaim 1 in which said first grinding means comprises a rod mill.

5. An apparatus as defined in claim l in which said additional grinding means comprises a ball mill.

6. An apparatus as defined in claim 1 including means for introducing water into said first grinding means, whereby the material discharged from said first grinding means is in the form of a slurry.

7. An apparatus as defined in claim 6 including a dewatering means interposed in the flow path between said screening device and said additional grinding means whereby to provide a controlled degree of dewatering of the slurry passing to said additional grinding means.

8. An apparatus as defined in claim 1 including crusher means adapted to reduce said mineral ore to the proper feed size for said first grinding means, and means for delivering the output of said crusher means to the input end of said first grinding means.

9. An apparatus for processing a mineral ore of a type having a desired mineral associated with a gangue material and a fibrous material associated with and constituting part of the ore, comprising a grinding means adapted to receive said mineral ore and to grind said mineral ore to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore, and a screening device adapted to receiver the ground ore including the liberated fibrous material discharged from said grinding means, said screening device having openings so sized as to retain on said screening device as oversize the liberated fibrous material and to pass through said screening device the defibered ore.

10. An apparatus as defined in claim 9 in which said mineral ore is nickel sulphide associated with serpentine gangue.

11. An apparatus as defined in claim 9 in which said fibrous material is asbestos.

12. An apparatus as defined in claim 9 in which said grinding means comprises a rod mill.

13. An apparatus as defined in claim 9 including means for introducing water into said grinding means, whereby the material discharged from said grinding means is in the form ofa slurry. v

14. An apparatus as defined in claim 9 including crusher means adapted to reduce said mineral ore to the proper feed size for said grinding means, and means for delivering the output of said crusher means to the input end of said grinding means.

15. The method of processing a mineral ore of the type in which a desired mineral is associated with a gangue material and in which the ore contains a fibrous material, comprising the steps of:

a. Supplying mineral ore of a suitable size to the input ofa grinding means;

b. Grinding said mineral ore in the grinding means to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore; and

c. Passing the discharge from said grinding means to a screening device having openings which are so sized as to retain the liberated fibrous material but which pass therethrough the defibered ore.

16. The method as defined in claim in which said grinding means is a rod mill.

17. The method as defined in claim 15 including the further additional steps of:

d. Passing the defibered ore to the input of additional grinding means; and

e. Grinding said defibered ore in said additional grinding means to a grind at which said desired mineral is liberated to a desired degree from the defibered ore.

18. The method defined in claim 15 in which said ore is nickel sulphide associated with serpentine gangue.

19. The method defined in claim 15 in which said fibrous material is asbestos.

20. The method defined in claim 17 in which said additional grinding means comprises a ball mill.

2]. The method defined in claim 15 in which the discharge passing from said grinding means to said screening device is in the form ofa fiber-laden slurry.

22. The method defined in claim 17 in which the defibered ore passing from said screening device to the input of said additional grinding means is in the form of a defibered slurry. I

he method defined in claim 22 including the step of providing a controlled degree of dewatering of the defibered slurry passing from said screening device to said additional grinding means.

24. A grinding circuit for processing a mineral ore of a type having a desired mineral associated with a gan gue material and a fibrous material associated with and constituting part of the ore, comprising a first grinding means adapted to receive said mineral ore and to grind said mineral ore to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore, and a screening device located downstream of said first grinding means and adapted to receive the ground ore including the liberated fibrous material discharged from said first grinding means, said screening device having openings so sized as to retain one said screening device as oversize the liberated fibrous material and to pass through said screening device the defibered ore.

25. A grinding circuit as defined in claim 24 in which said mineral ore is nickel sulphide associated with serpentine gangue.

26. A grinding circuit as defined in claim 24 in which said fibrous material is asbestos.

27. A grinding circuit as defined in claim 24 in which said first grinding means comprises a rod mill.

28. A grinding circuit as defined in claim 24 including means for introducing water into said first grinding means, whereby the material discharged from said first grinding means and passing to said screening device is in the form ofa slurry.

29. A grinding circuit as defined in claim 24 including additional grinding means located downstream of said screening device and adapted to receive the defibered ore passing through said screening device, said additional grinding means being effective to liberate said desired mineral to a desired degree from the defibered ore.

30. A grinding circuit as defined in claim 29 in which said additional grinding means comprises a ball mill.

3]. A grinding circuit as defined in claim 28 including additional grinding means located downstream of said screening device, and further including a dewatering means interposed in the slurry flow path between said screening device and said additional grinding means whereby to provide a controlled degree of dewatering of the slurry passing to said additional grindin g means. 

2. An apparatus as defined in claim 1 in which said mineral ore is nickel sulphide associated with serpentine gangue.
 3. An apparatus as defined in claim 1 in which said fibrous material is asbestos.
 4. An apparatus as defined in claim 1 in which said first grinding means comprises a rod mill.
 5. An apparatus as defined in claim 1 in which said additional grinding means comprises a ball mill.
 6. An apparatus as defined in claim 1 including means for introducing water into said first grinding means, whereby the material discharged from said first grinding means is in the form of a slurry.
 7. An apparatus as defined in claim 6 including a dewatering means interposed in the flow path between said screening device and said additional grinding means whereby to provide a controlled degree of dewatering of the slurry passing to said additional grinding means.
 8. An apparatus as defined in claim 1 including crusher means adapted to reduce said mineral ore to the proper feed size for said first grinding means, and means for delivering the output of said crusher means to the input end of said first grinding means.
 9. An apparatus for processing a mineral ore of a type having a desired mineral associated with a gangue material and a fibrous material associated with and constituting part of the ore, comprising a grinding means adapted to receive said mineral ore and to grind said mineral ore to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore, and a screening device adapted to receive the ground ore including the liberated fibrous material discharged from said grinding means, said screening device having openings so sized as to retain on said screening device as oversize the liberated fibrous material and to pass through said screening device the defibered ore.
 10. An apparatus as defined in claim 9 in which said mineral ore is nickel sulphide associated with serpentine gangue.
 11. An apparatus as defined in claim 9 in which said fibrous material is asbestos.
 12. An apparatus as defined in claim 9 in which said grinding means comprises a rod mill.
 13. An apparatus as defined in claim 9 including means for introducing water into said grinding means, whereby the material discharged from said grinding means is in the form of a slurry.
 14. An apparatus as defined in claim 9 including crusher means adapted to reduce said mineral ore to the proper feed size for said grinding means, and means for delivering the output of said crusher means to the input end of said grinding means.
 15. The method of processing a mineral ore of the type in which a desired mineral is associated with a gangue material and in which the ore contains a fibrous material, comprising the steps of: a. Supplying mineral ore of a suitable size to the input of a grinding means; b. Grinding said mineral ore in the grinding means to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore; and c. Passing the discharge from said grinding means to a screening device having openings which are so sized as to retain the liberated fibrous material but which pass therethrough the defibered ore.
 16. The method as defined in claim 15 in which said grinding means is a rod mill.
 17. The method as defined in claim 15 including the further additional steps of: d. Passing the defibered ore to the input of additional grinding means; and e. Grinding said defibered ore in said additional grinding means to a grind at which saId desired mineral is liberated to a desired degree from the defibered ore.
 18. The method defined in claim 15 in which said ore is nickel sulphide associated with serpentine gangue.
 19. The method defined in claim 15 in which said fibrous material is asbestos.
 20. The method defined in claim 17 in which said additional grinding means comprises a ball mill.
 21. The method defined in claim 15 in which the discharge passing from said grinding means to said screening device is in the form of a fiber-laden slurry.
 22. The method defined in claim 17 in which the defibered ore passing from said screening device to the input of said additional grinding means is in the form of a defibered slurry.
 23. The method defined in claim 22 including the step of providing a controlled degree of dewatering of the defibered slurry passing from said screening device to said additional grinding means.
 24. A grinding circuit for processing a mineral ore of a type having a desired mineral associated with a gangue material and a fibrous material associated with and constituting part of the ore, comprising a first grinding means adapted to receive said mineral ore and to grind said mineral ore to substantially the coarsest grind at which most of the fibrous material is liberated from the rest of the ore, and a screening device located downstream of said first grinding means and adapted to receive the ground ore including the liberated fibrous material discharged from said first grinding means, said screening device having openings so sized as to retain on said screening device as oversize the liberated fibrous material and to pass through said screening device the defibered ore.
 25. A grinding circuit as defined in claim 24 in which said mineral ore is nickel sulphide associated with serpentine gangue.
 26. A grinding circuit as defined in claim 24 in which said fibrous material is asbestos.
 27. A grinding circuit as defined in claim 24 in which said first grinding means comprises a rod mill.
 28. A grinding circuit as defined in claim 24 including means for introducing water into said first grinding means, whereby the material discharged from said first grinding means and passing to said screening device is in the form of a slurry.
 29. A grinding circuit as defined in claim 24 including additional grinding means located downstream of said screening device and adapted to receive the defibered ore passing through said screening device, said additional grinding means being effective to liberate said desired mineral to a desired degree from the defibered ore.
 30. A grinding circuit as defined in claim 29 in which said additional grinding means comprises a ball mill.
 31. A grinding circuit as defined in claim 28 including additional grinding means located downstream of said screening device, and further including a dewatering means interposed in the slurry flow path between said screening device and said additional grinding means whereby to provide a controlled degree of dewatering of the slurry passing to said additional grinding means. 